Riding on the success of digital coherent technology in 100G systems deployment, optical quadrature amplitude modulation (QAM) with larger signal constellation maps, such as 16-QAM or 32-QAM, has become the main theme in optical communication research for the quest to achieve the next channel capacity such 400 Gb/s or 1 Tb/s. By using a larger QAM constellation maps, channel spectral efficiency can be improved from the current 100G DP-QPSK technology so that larger fiber data capacity can be supported. To generate optical QAM, typically two streams of multiple-level electrical baseband signal will be used to drive the two arms (in-phase and quadrature) of an integrated nested Mach-Zehnder modulator (MZM). In some cases such as laboratory evaluation purpose, however, the two arms are driven using the same data copies, or data copies with opposite polarities, with RF de-correlation delay lines to save cost. The commercially available RF de-correlation delay lines are inherently bulky, have bandwidth limitation (40˜50 GHz), and have slow frequency roll-off response which could worsen and affect the signal integrity when the cable length increases. If the QAM signal baud-rate is large, another factor to consider is the bandwidth of the optical modulator. Typically, integrated nested MZMs, or optical I/Q modulators, have lower bandwidths compare to regular intensity MZMs due to its design where other parameters such as I/Q balancing and size limitations. Commercial I/Q modulators typically have bandwidths below 25 GHz with very few reaching more than 30 GHz. Therefore, it is still very difficult to evaluate system performance for 400 Gb/s DP-16QAM channel using just a single optical modulator.
Kylia, a fiber-optic device manufacturer for the telecom networks using free-space optic technology, offers two commercial products for optical QAM signal (16-QAM or 64-QAM) emulation. Both emulators utilize free-space optical delay interferometer technique and require optical QPSK signals to be used as inputs. Sine optical I/Q modulator needs to be used for QPSK signal generation, the signal baud-rate will be limited by the I/Q modulator bandwidth and the RF cables if I/Q data de-correlation is used. Moreover, their scheme requires very precise amplitude and phase control on the multiple optical paths in order to obtain the correct QAM constellation at the interferometer output with no simple way to make phase adjustments other than detecting and recovering the QAM constellation.
ZTE Corporation's converting optical QPSK to 16-QAM signal method involves carving the transition edge of an optical NRZ QPSK signal, which has four distinctive amplitude levels at each quadrature. The produced result will resemble a 16-QAM constellation, however the adjacent symbols are highly correlated and therefore unsuitable for actual performance analysis. Also, optical I/Q modulator and RF delay line for de-correlation are required.